1. Field of Use
This invention relates generally to die assemblies for punching holes through stacks of paper sheets. In particular, it relates to improvements in the construction and mode of operation of such die assemblies and components therefor.
2. Description of the Prior Art
Some publications and notebooks comprise a plurality of paper sheets arranged in a stack, front and back paperboard covers at opposite end faces of the stack and a spiral binder, in the form of a metal or plastic helical coil, which is threaded through a row of closely spaced holes provided along an edge of the the stack and along the corresponding edges of the covers.
One step in the manufacture of such publications and notebooks involves insertion of the stack of sheets in a die assembly in a die press and operating the latter to punch a row of holes in the stack. The die assembly, which typically comprises a stationary die block with a row of die holes therein and a relatively movable punch retainer plate, supporting a row of elongated steel punches, which is detachably mounted on the frame of the die press. The punch retainer plate and the punches thereon are reciprocably moved as a unit relative to the die block by a movable drive member on the press between open and closed positions. The drive member is motor-driven or, in the case of small presses, manually operated.
The die assembly typically comprises four principal components, namely: the aforementioned stationary lower die block, a stationary guide plate located above and spaced from the die block by a stationary separator plate, the aforementioned reciprocably movable upper punch retainer plate which is located above the guide plate, and the aforementioned row of downwardly extending punches which are rigidly secured to and movable with the punch retainer plate. The space between the lower die block and the guide plate serves as a stack-slot which is adapted to receive the edge of a stack of paper sheets which is to have a row of holes punched therethrough. The die block, guide plate and punch retainer plate are each provided with a row of die holes, guide holes and punch-retainer holes, respectively. The punch retainer holes are part of a means for fixedly securing the punches on the punch retainer plate. The guide holes guide the punches into alignment with the die holes. The die holes cooperate with the punches as the stack is pierced and serves as an exit for punched-out paper scraps. Each hole corresponds in shape to the transverse cross-sectional configuration of the punch receivable therein which, for example, may be round, retangular or any other desired configuration, depending on the desired punched hole shapes for the stack. Each punch extends downwardly through and is slidably engaged by a registering guide hole in the guide plate. The lower cutting end of each punch is reciprocably movable through the stack-slot (and through any stack therein) and through a registering die hole in the die block as the punch retainer plate and punch thereon are reciprocably moved by the drive member of the press.
In one cycle of operation, the upper punch retainer plate and punches thereon are initially in open or raised position. The edge of a stack of paper sheets is inserted into the stack-slot. Then, the press is operated so that its drive member moves the upper punch retainer plate and punches thereon downwardly as a unit to closed position. As this occurs, the row of punches penetrate the stack and form a row of holes therein. The cutting ends of the punches then enter and slidably engage the row of die holes in the lower die block to effect expulsion of the punched-out pieces of paper through the open lower ends of the die holes. Thereafter, the drive member moves the upper punch retainer plate and punches thereon upwardly as a unit to open position so that the punches retract into the guide holes and are clear of the stack-slot so that the punched stack can be removed from the stack-slot, whereupon the die assembly and press are in readiness for the next cycle of operation.
Prior art die assemblies of the aforesaid character are subject to considerable wear in the course of prolonged, repetitious, high-speed operation. Therefore, they require relatively frequent and costly maintenance, servicing and component replacement. Most wear occurs at the cutting ends of the punches, at the upper end of the row of die holes in the die block and in the guide holes in the guide plate. The cutting edges of the punches must be periodically sharpened by regrinding (which gradually reduces their original length), as do the upper edges of the die holes, although not as often as the punches (a typical ratio for service being about 5 to 1). In prior art die assemblies wear is attributable to the particular design of the die assembly and to the particular methods of manufacture of the components thereof. However, efforts aimed at avoiding or mitigating certain wear problems have resulted in die assembly designs which merely substitute one set of problems and remedial procedures for another, with no net reduction in maintenance, service and component replacement costs.
For example, during manufacture of some prior art die assemblies, in an effort to reduce the number of individual hole drilling operations, the metal blanks for the die block, guide plate and punch retainer plate are stacked one on top of the other, clamped together and then drilled, using one or more drill bits. In this arrangement, each drill bit forms a set of three holes, each set including a die hole, guide hole and punch-retainer hole. However, this attempt at production efficiency has the following drawbacks. First, the drill bit tends to bend and then slant as it cuts into blanks (which are typically made of different metals) and cannot be maintained precisely perpendicular to the blanks. As a result, the holes in a set are slightly misaligned from each other when drilled and this misalignment becomes amplified when the three plates are mounted in spaced-apart relationship in the finished die assembly. Second, each set of holes cannot be drilled so as to be precisely parallel to an adjacent (or other) set of holes. Therefore, further random misalignment results, which is also exagerated when the plates are spaced apart in the finished die assembly. The end result is that each punch rigidly and fixedly installed in a punch-retainer hole may itself be slanted and, in most cases, is not precisely aligned with its associated guide hole and die hole. This causes substantial wear during die operation.
Attempts to overcome this problem of misalignment involved fabrication of the guide plate of relatively soft metal, such as bronze, to reduce friction and wear between the guide holes and the steel punches. Furthermore, the die block was fabricated of relatively soft steel, so that when the edges of die holes became worn, as a result of friction with the steel punches, the edges of the die holes could be peened slightly closed. Thereafter, the steel punches were then relied on to punch through the peened edges to re-cut the die holes with no clearance. Of course, this imposed additional and undesirable wear on the cutting edges of the punches themselves and required further re-grinding. Obviously, the prior art solutions are limited in effectiveness, are time-consuming, labor intensive, costly and do not contribute to component life.